Projections of future stream water chemistry under the Representative Concentration Pathways in an old-growth Douglas-fir forest of the Pacific Northwest using a biogeochemical model with improved algorithm on soil cation exchange capacity
Abstract
Stream water chemistry of old-growth forests in the Pacific Northwest are characterized by relatively high pH and acid-neutralizing capacity (ANC). Under future climate change, concentrations of base cations are expected to play important roles in regulating stream water chemistry of the region. Previous sensitivity analysis of a biogeochemical model, PnET-BGC, demonstrated the importance of uncertainty associated with soil cation exchange capacity (CEC) to both soil and stream water chemistry in the Great Smoky Mountains and the Pacific Northwest. In this study, we improved the model algorithm used to calculate soil CEC by relating the parameter with soil organic matter content. Future projections of soil and stream water chemistry were made under two Representative Concentration Pathway (RCP) scenarios at Watershed 2 of the H. J. Andrews Experimental Forest in Oregon, a 500 years old-growth forest dominated by Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla).
Using the RCP scenarios statistically downscaled from four General Circulation Models (GCMs), we project a greater than 40% decrease in CEC at the watershed under the RCP4.5 and RCP8.5 scenarios due to significant decreases in carbon inputs to the soil through foliar, wood, and root litter as a result of heat-induced tree mortality. These decreases in CEC and a decoupling of the nutrient supply from mineralization with uptake by plants are projected to result in marked increases in stream water concentrations of Ca2+, Mg2+, K+, and Na+. In addition, soil base saturation, stream water pH and ANC also showed great future increases, facilitating the sequestration of atmospheric CO2 in stream water. Projected changes in stream water quality in the old-growth forest in the Pacific Northwest are compared with simulations from another Long Term Ecological Research site, Hubbard Brook Experimental Forest, in the northeastern United States under the RCP4.5 and RCP8.5 scenarios.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B21I2447D
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCESDE: 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCESDE: 1879 Watershed;
- HYDROLOGY